The invention relates generally to tracking filters for eliminating out-of-band noise in radar systems and specifically to a frequency mobile narrow band filter used to move and lock a Bulk Acoustic Resonator's response to the frequency of incoming signals.
Bulk Acoustic Resonators have a history of poor thermal stability with their responses changing critically with comparatively mild changes of thermal conditions. In one instance, a particular Bulk Acoustic Resonator's response signal was observed to move down approximately 60 KHz for every 1.degree. C. rise in temperature. With an operating bandwidth of 40 KHz there resulted a strong danger of the complete loss of the tracking signal.
The problem of thermal effects on frequency responses in radar teaching components has been alleviated to some degree by the prior art techniques given by the following patents:
U.S. Pat. No. 4,074,213 issued to Epsztein et al on Feb. 14, 1978;
U.S. Pat. No. 4,320,365 issued to Black et al on Mar. 16, 1982;
U.S. Pat. No. 3,170,120 issued to Jensen et al on Feb. 16, 1965;
U.S. Pat. No. 3,868,605 issued to Poole on Feb. 25, 1975; and
U.S. Pat. No. 3,349,348 issued to Ice on Oct. 24, 1967.
Epsztein et al disclose an elastic bulk wave used to filter an electrical signal having a plurality of frequency components. The patented device forms an acoustic bulk wave filter for high frequency oscillator applications. A bulk acoustic wave resonator and filter construction is shown in Black et al. Jenson et al disclose a comb filter and Poole is directed to an adaptable notch filter. Temperature compensation for a quartz crystal filter is suggested in the Ice patent.
While the above prior art techniques are instructive, the problem remains for Bulk Acoustic Resonators having a very narrow band of frequency response coupled with changes in frequency in incoming signals. In view of the foregoing discussion, it is apparent that there currently exists the need for moving the response comb of the Bulk Acoustic Resonator up or down in accordance with changes in the frequency of the incoming signals. The present invention is directed towards satisfying that need.